The present invention relates generally to alternating current (AC) induction motors and, more particularly, to a system and method for controlling operation of a motor drive during fast start-up of an induction motor.
The usage of motor drives in various industries has become more prevalent based on the increasing need for energy savings and control flexibility in operation of induction motors. Based on these needs, improvements in motor control performance have become increasingly important. One such example of a need for improved motor control performance is when fast start-up of an induction motor is desired. When operating an adjustable-speed motor drive (ASD) to start an induction motor, users have options to set an acceleration time for bringing the motor up to a desired reference speed. This acceleration time can be as fast as 0.1 s from the motor's zero speed to the desired speed.
However, several issues/drawbacks are inherent when performing such a fast start-up of the induction motor. For example, due to inherent rotor inertia (as well as load inertia), a slip occurs in the motor during the acceleration process. If the system inertia, and the corresponding motor slip, is too large, an over-current phenomenon can occur that causes an associated over-current trip fault in the motor. Another issue/drawback associated with fast start-up of the induction motor is the possibility for an over-voltage occurrence where the induction motor is caused to operate in a power generation mode. That is, at the end of the acceleration process, at the moment when the actual speed of the induction motor reaches the desired speed reference set point, the motor current will not change immediately due to the existence of stator inductance in the motor. The electromagnetic torque is thus still larger than the load torque and this causes the actual speed of the motor to continue to rise to a level above its reference speed, thereby causing the induction motor to operate in a power generating mode. The energy stored in the induction machine will be fed back through the inverter of the ASD, such that a DC link voltage of the ASD is boosted. The boosted voltage present on the DC link may cause an over-voltage trip in the ASD when an over-voltage threshold is reached.
The over-current and over-voltage trip faults that can occur during start-up of the induction motor are undesirable. Such over-current and over-voltage trip faults can cause delays in bringing the motor up to the desired speed and can disrupt the power production process.
It would therefore be desirable to design a system and method for controlling operation of an ASD during fast start-up of an induction motor. It would further be desirable for such a system and method to provide for achieving a smooth start-up of the induction machine without disrupting the operation thereof due to over-current and over-voltage trip faults.